FIG. 1 shows a basic diagram of a state of the art LED driver circuit that is used, for example, in an image reading apparatus in conjunction with a CIS or CCD image sensor device. An LED array L1-L3 is coupled to switches S1-S3 and resistors R1-R3. The LED array L1-L3, switching circuit S1-S3, and resistors R1-R3 are arranged separately from the analogue front end (AFE) circuitry that is used to process image data received from the sensor device, such as a Photo Diode Array (PDA) for example.
In a colour image scanner, the LED array typically comprises Red (L1), Green (L2) and Blue (L3) LEDs. Light of each colour emitted from the respective LED illuminates a portion of a target object that is to be scanned, and the light reflected by the object is incident on a sensor device. Typically, the sensor device comprises an array of sensors arranged linearly as a line image sensor, each element of the sensor array comprising a photoelectric conversion element, such as a photodiode and a capacitor for each pixel, which converts incident light into a current which is accumulated as a charge on the capacitor. The respective charges accumulated on the respective capacitors are converted into respective voltages that are then output from the sensor array (PDA).
The voltages output from the PDA are converted by an Analogue to Digital Converter (ADC) into digital signals for subsequent processing during generation of the image of the target object being scanned.
The scanning of an image is usually performed using a line scanning operation. For colour images, each line is scanned by the Red, Green and Blue light sources. That is, the Red LED L1 is turned on to read one line in a scanning direction, thereby obtaining the Red component of that line. The Green LED L2 is then turned on to obtain the Green component of that line, followed by the Blue LED L3 being turned on to obtain the Blue component of that line. The LED array and sensor array are then typically moved on a carriage mechanism to align with the next line on the target object. Each LED L1-L3 is turned on by switching on the respective switch S1-S3, using respective switch control signals CS1-CS3 received from a switch controller logic circuit SC.
While one line is being scanned, image data received from the sensor array (PDA) relating to a previous line scan is read out serially and processed by the ADC.
The current flowing through each LED is defined by the current-voltage characteristics of the LED, by the resistance of the series resistor and by the voltage applied from the power supply, PSU, (the on-resistance of the switch usually being negligible).
FIGS. 2a-2d show the switch control signals CS1-CS3 and the supply current IS drawn from the power supply PSU in a “constant current” mode. The ground current is substantially equal to the supply current.
In addition to each LED passing a respective constant current during illumination of the target object, it is also known to use Pulse Width Modulation (PWM) control signals for controlling the illumination of each LED, such that the illumination or intensity of the LED can be controlled by controlling the duty-cycle and/or frequency of the PWM control signals. The current though the LED may be subject to wide variation due to tolerances of the power supply voltage and the (temperature-dependent) I-V characteristic of the LEDs.
FIGS. 2e-2h show the switch control signals CS1-CS3 and the supply current IS drawn from the power supply in such a PWM mode.
It will be appreciated that, in both the constant current and PWM modes of operation, the quality of the image data received from the PDA 3 is related to the intensity at which each LED L1-L3 is illuminated during respective scan periods. In addition, it is noted that the brighter the LED is illuminated, the less the sampling (i.e. integration), and hence scanning, time is required which means that the scanner can operate quicker. Therefore, it is desirable to operate the LEDs at or near their maximum current ratings without damaging the LEDs.
According to one known system, the illumination of an LED L1-L3 is controlled by passing a predetermined current through the LED, the predetermined current being chosen according to known characteristics of the LED, power supply or switch resistance. However, due to the above mentioned variations caused by tolerances of the power supply voltage and the (temperature-dependent) I-V characteristic of the LEDs, setting a predetermined maximum current in this way does not enable an LED to be illuminated at its absolute maximum intensity, since some degree of safety margin must be incorporated to allow for such tolerances. If this safety margin in made small (i.e. in order to obtain the maximum possible current), then the possibility of damaging an LED is increased, i.e. due to an over-current being passed through the LED.
It is also known to illuminate an LED L1-L3 by directly monitoring the amount of current flowing through the LED, and adjusting the current flow accordingly such that it operates near its maximum intensity. This involves monitoring the actual current flowing through LED. Operating an LED near its maximum intensity in this way can also result in the LED being damaged by an over-current, particularly when switching from one LED to another. In other words, when switching from one LED to another, if the initial current exceeds the maximum rating of the LED, then the LED will be damaged before the current monitor can detect and adjust the current flow.
It is therefore an aim of the present invention to provide a protection circuit and method for protecting a light source such as an LED, without having the disadvantages mentioned above.